Dither tuner for a coaxial magnetron



Dec. 3, 1968 GLENFlELD 3,414,761

DITHER TUNER FOR A COAXIAL MAGNETRON Filed Sept. 16, 1965 INVENTOR. GEORGE asuzunaw ATTORNEY United States Patent Oflice 3,414,761 Patented Dec. 3, 1968 3,414,761 DITHER TUNER FOR A COAXIAL MAGNETRON George E. Glenfield, Scotch Plains, N.J., assignor to S-F-D Laboratories, a corporation of New Jersey Filed Sept. 16, 1965, Ser. No. 487,697 4 Claims. (Cl. 315--39.61)

ABSTRACT OF THE DISCLOSURE A dither tuned coaxial magnetron is disclosed. The caxial magnetron tube includes an evacuated envelope structure containing an anode electrode structure having a circular array of coupled anode resonators coaxially disposed of a circular electric mode cavity resonator coupled to the anode resonators via an array of coupling slots. The coupling slots serve to lock the 1r mode of the anode resonators to the circular electric mode of th cavity resonator. An axially translatable end wall of the circular electric mode resonator is provided for tuning the resonant frequency of the cavity resonator and the coupled anode resonators for tuning the output frequency of the tube. A cathode electrode structure is coaxially disposed of the anode to produce a magnetron interaction region therebetween. An elongated tuner actuating shaft is coupled to the movable tuning wall and extends through the vacuum envelope of the tube via a vacuum tight bellows. A motor driven crankshaft is coupled via a yoke to the outer end of the tuner actuating shaft for dithering of the shaft and its dependent tuner at a relatively high rate as of 20 to 160 cycles per second to produce frequency modulation of the output frequency of the tube. The motor is affixed to the outside of a housing and the housing is filled with a viscous fluid such as oil. The oil surrounds the crankshaft, yoke, and tuner actuating shaft for dampening the oscillatory motion of the tuner.

High power, high frequency tubes, such as crossed field microwave tubes or magnetrons, are generally employed in radar scan systems for detecting ground clutter, scatter and the like. To differentiate signals reflected from various objects, the magnetron is modulated or dithered by means of a tuner. Modulation is achieved in one approach by the axial movement of a tuning rod and disk within the cavity of the magnetron to effectuate a variable displacement of the radio frequency magnetic field lines. One such type tube is described in copending U.S. patent application Ser. No. 221,796, filed Sept. 6, 1962, now issued as U.S. 3,223,876 on Dec. 14, 1965, and assigned to the same assignee.

For accurate observation and detection, it is most desirable that the frequency modulated transmitted signal be precise and readily controllable over a desired range of frequency. However, any misalignment of the tuning rod, or variations in the axial oscillation of the rod results in undesirable changes in the modulating frequency. Also, parasitic modes of oscillation may appear, with resultant reduction in tube power output at desired frequencies.

Furthermore, some known apparatus incorporating tuner assemblies utilize hydraulic drive with oil flow through the system. After some use, the oil seals in the system wear, allowing oil leaks. As 'a result, there is a reduction in pressure with erratic fluctuations in the modulation frequency. Therefore, compensating circuits are needed to take care of the loss of oil and pressure. Also, other mechanical parts such as pistons begin to wear and necessitate replacement. Thus, such known apparatus are expensive to construct and maintain. Besides,

any shutdown due to inoperability of the crossed field device may disable the security detection system in which it is employed, which is a highly undesirable condition.

An object of this invention is to provide a novel and imbproved tuning means for a high frequency, high power tu e.

Another object of this invention is to provide an improved high rate of change mechanical tuner for a magnetron.

Another object is to provide a tuner mechanism for a magnetron that affords precise tuning and longer life.

According to this invention, the tuning means of a magnetron tube comprises a mechanical assembly including an input shaft coupled to a drive means, a connecting rod connected to such input shaft and disposed along the axis of the tube, and a tuning plunger and disk coupled to the rod. The tuning disk is oscillated in the vicinity of an annular anode electrode such that the frequency output of the tube is modulated at a rate determined by the frequency of oscillation of the disk along the tube axis. Precision mechanical alignment and retainer means are utilized to maintain the tuning disk in precise alignment with the tube axis and coaxially with the annular anode, whereby the modulated frequency remains effectively constant during operation.

The invention will be described in greater detail with reference to the drawing in which:

FIG. 1 is a perspective view of the inventive crossed field device or magnetron assembly; and

FIG. 2 is a fragmentary cross-section view of the assembly, taken along lines 22 of FIG. 1.

In FIGURE 1, a crossed field device, such as a. magnetron, comprises a support or mount 10 incorporating a permanent magnet structure for providing a unidirectional magnetic field. The support 10 contains a hollow body 12, made of copper for example, adapted for evacuation. A waveguide assembly 14 is connected to the body 12 through a portion of the support 10. At an opposing portion of the support 10, a tuner assembly 16 is disposed in accordance with this invention. A cathode lead-in structure 18 extends from the support 10 in quadrature relationship with the axially aligned output waveguide 14 and tuner assembly 16 in the conventional manner as is well known from the aforecited U.S. Patent 3,223,876.

In the detailed cross-section of FIGURE 2, the magnetron of FIGURE 1 includes a cavity structure 20 wherein a cathode electrode 21 and "an annular output electrode or anode resonator structure 22 are disposed in coaxial alignment. The cavity structure 20 comprises a circular mode cavity which is disposed centrally of the anode structure 22 along the axis of the tube. An array of anode resonator vanes 23 surround the circular electric mode cavity 20 and the adjacent vanes have spaces therebetween. Alternate anode resonators are electromagnetically coupled to the circular electric mode cavity 20 by means of axially directed slots 24 communicating through a common wall between the anode resonators and the cavity 20. A magnetron interaction region surrounds the outer tips of the anode vanes 23 and is defined by the space in between the vanes and the surrounding emitter ring of the cathode 21.

The cathode lead-in assembly 18 not shown in FIG. 2, is connected internally of the tube via electrical leads to the cathode 21 and enables the formation of electrons that interact with crossed electric and magnetic fields in the region of the anode resonators. For this purpose, a strong axial DC magnetic field, of about 12,000 to 15,000 gauss by way of example, is provided by means of a permanent magnet encompassing the anode electrode 22. The magnetron interaction region is disposed between a pair of magnet pole pieces 25 positioned on opposite sides of the anode electrode. With this type configuration, the magnetron produces a radio frequency output of about 32-37 gigacycles per second (go/sec.) 'Which is directed to a utilization load through the output waveguide 14 or a coupling loop, in a well known manner.

In accordance with the present invention, a tuner mechanism 16 is coupled to the cavity structure 20 for achieving modulation of the characteristic resonant frequency of the cavity. The tuner mechanism includes an input shaft or crankshaft 26 driven by a motor drive means 28, which may be rotated at 9600 revolutions per minute, for example. The crankshaft 26 is engaged by eccentric coupling to a connecting rod 30 that is actuated to move along the central axis of the magnetron tube. The eccentric coupling includes a yoke 29 riding on an eccentric portion of the crankshaft 26. To compensate for any longitudinal thrust of the crankshaft 26, an assembly including resilient rings 32 and 34 are provided adjacent to the shaft 26. The O-ring 32 at the terminus of the crankshaft is secured by a hardened steel washer 36, which in turn is urged against the O-ring 32 by a ballpoint contact 38 secured in a groove of the shaft 26. The O-ring assembly 32 affords counterthrust to any erratic longitudinal travel of the shaft 26.

The O-ring 34 is located adjacent to the shaft 26 and applies pressure to a thin walled Teflon (Du Pont product) sleeve 56. This combination allows high speed rotation of shaft 26 without deterioration of the oil seal created by O-ring 34. Preloaded ball bearings 40 are also secured within the yoke 29 adjacent the crankshaft 26 to avoid radial displacement and to maintain precise alignment of the shaft.

To reduce mechanical friction, damp spurious vibrations and enhance life, the connecting rod 30 is encompassed by a lubricating, viscous oil 42. Oil seals 44 are provided to prevent seepage of the oil 42 out of the chamber enclosing the connecting rod 30.

The connecting rod 30 is linked, by threaded adaptor means for example, to a tuning plunger 46 that carries a tuning disk 48, disposed within the cavity structure 20a and adjacent to the anode resonator structure 22. The diameter of the rod 30 is relatively small, affording flexibility of the rod, so that only linear axial motion is translated to the plunger 46. The plunger 46 is vacuum sealed to the envelope of the tube by means of a cylindrical bellows 50. The plunger 46 may be of tungsten, and the disk 48 may be of copper, by way of example. The disk may be /32 inch thick and have a diameter of 0.340 inch, for example. For maintaining axial alignment of the timing plunger 46, which is coaxially aligned with the annular anode 22, a double bearing assembly 52 engages the plunger assembly in rigid fashion, allowing only linear motion.

In operation, the tuning disk 48 is moved axially when the drive means 28 is energized. The displacement stroke of the disk 48 is about 0.003 inch and the disk oscillates at about 160 cycles per second. As a result, a modulation of approximately 150 megacycles per second (mc./sec.) is realized, which is a mc. deviation of the Ka band frequency of 32-37 gc./sec. for the magnetron tube employed. This modulation may be varied about the center frequency at a 20-160 c.p.s. rate.

If it is desired to change the 150 mc./sec. modulation frequency to another frequency, e.g., l0 mc./ sec. then a simple replacement of the crankshaft 26 may be made with a shaft of another size.

A frequency readout device 54 may be coupled to the crankshaft 26 for translating the frequency of rotation, and thus the frequency of oscillation of the plunger 46 and disk 48, to a frequency value related to the tube frequency.

The frequency readout device 54 may be a crystal serving as a pressure transducer or voltage transducer that measures the degree of rotation, or the change of position or phase of the input shaft 26. The sensed change of phase is translated to a readout frequency representing modulation frequency. Alternatively, the readout device may be coupled to the rotary drive means 28 in lieu of the input shaft. Also, a gated photocell or linear voltage device may be coupled to the rotary shaft 26 for frequency readout.

By means of the inventive assembly described above, wherein a mechanical plunger assembly is disposed in combination with a crossed field device, an improved and economical technique for frequency modulation of a precisely tuned high frequency, high power tube is made possible. The simple, inexpensive mechanical assembly affords precision alignment of the several parts, and realizes long life by virtue of the arrangement of bearings, oil lubrication and oil seals.

What is claimed is:

1. In a coaxial magnetron, means forming an evacuated envelope structure, means forming an anode structure having a circular array of coupled anode resonators disposed inside said envelope, means forming a circular electric mode cavity resonator coaxially disposed of said array of anode resonators, means forming an array of electromagnetic coupling slots communicating between said circular electric mode cavity resonator and said array of anode resonators for locking the 1r mode of the anode resonators to a circular electric mode of said circular electric mode resonator, means forming an axially translatable end 'wall of said circular electric mode resonator for tuning the resonant frequency of the coupled resonator system which includes said circular electric mode resonator and said array of anode resonators coupled thereto, means forming a cathode electrode structure coaxially disposed of said array of anode resonators for defining an annular magnetron electromagnetic interaction region therebetween, the improvement comprising, means for oscillating said cavity end wall structure for frequency modulating the output wave energy of the coaxial magnetron and including, an elongated drive shaft coupled to said movable cavity tuning wall and extending axially of the tube, means forming a rotatable crankshaft having an eccentric portion forming a crank portion of said crankshaft, said crankshaft extending transversely of said drive shaft, an electric motor for rotatably driving said crankshaft, means forming a yoke for coupling said crankshaft to said drive shaft for translating rotary motion of said crankshaft into oscillatory motion of said drive shaft and dependent cavity tuner, and means forming a bellows structure vacuum sealing said drive shaft to said vacuum envelope.

2. The apparatus of claim 1 including a housing external of said vacuum envelope and enclosing said yoke, crankshaft and an outer end of said drive shaft, and a viscous liquid filling said housing and surrounding said yoke, crankshaft and drive shaft for dampening the oscillatory motion of said drive shaft.

3. The apparatus of claim 1 wherein said motor drives said drive shaft at oscillatory speeds in excess of cycles per second.

4. The apparatus of claim 1 wherein said movable cavity tuning wall comprises a conductive disc.

References Cited UNITED STATES PATENTS 3,289,035 11/1966 Drexler 3l539.6l X

OTHER REFERENCES New Magnetron Shifts Frequency Fast, by Edwards Electronics, April 6, 1964, TK 7800, E 58, 6 pages of specifications and drawings. Copy in Group 250, 315- 39.51.

HERMAN KARL SAALBACH, Primary Examiner. S. CHATMON, JR., Assistant Examiner. 

